Method for obtaining the image of the internal structure of an object

ABSTRACT

This invention relates to nondestructive testing of the internal structure of objects. 
     The method for obtaining the image of the internal structure of an object consists in that the flux of a penetrating radiation emitted by a source (1) is collimated by a single-crystal monochromator (4), an object (7) is irradiated with the thus-collimated radiation, the radiation transmitted through the object (7) is collimated by a single-crystal (8) and then registered by a detector (9). According to the invention, use is made for irradiating the object, of an asymmetrical reflection of the radiation from the single-crystal monochromator (b 4), a pseudosheet-like beam (5) is formed, having a wide wavefront and a divergence in the Bragg diffraction plane at least twice as little as the width of the Bragg reflection of the single-crystal (8), which is set in the position of Bragg reflection with respect to the pseudosheet-like beam (5) formed by the single-crystal monochromator (4). 
     The method is aimed preferable, for diagnosing polymer and ceramic products, as well as for medical diagnosis.

TECHNICAL FIELD

This invention relates generally to methods for nondestructive testing of objects using penetrating radiation and has particular reference to methods for obtaining the image of the internal structure of an object.

BACKGROUND ART

One state-of-the-art method for obtaining the image of the internal structure of an object is widely known to comprise irradiation of the object under test with an X-ray beam and recording the radiation that has passed through the object with the aid of a detector, e.g., an X-ray sensitive film. Useful information appears, after decoding X-ray photographs, as data on the amount of radiation absorbed by the areas of the object under test having different degrees of radiotranslucency.

However, X-rays used in the heretofore-known method is a continuous X-ray spectrum polychromatic radiation. This offers some difficulties in interpretation of the results obtained, inasmuch as the long-wave spectral components are as a rule absorbed by the object itself and the proportion of absorbed radiation varies from one object to another, which cannot be allowed for at all times. This in turn results in adversed reliability of the results of inspection of an internal structure of the object under test.

One more state-of-the-art method for obtaining the image of the internal structure of an object is known to provide irradiation of the object under test with an X-ray beam collimated by a single-crystal monochromator, followed by recording the radiation passed through the object in a detector (W. A. Ellingson, M. W. Vanier "Application of Dual-energy X-ray Computed Tomography to Structural Ceramics", Adv., X-ray, Anal., 1988, v. 32, pp. 629-640).

The method mentioned above provides for higher sensitivity and reliability of the results of inspection due to the use of monochromatic radiation.

However, the method in questions is practically unsuitable, as the one discussed before, for testing the internal structure of objects featuring weak absorption of a penetrating radiation, especially such portions of the object under test, wherein the degree of X-ray transparency is close to that of the surrounding medium.

Finally, a method for obtaining the image of the internal structure of an object is also known, consisting in that the flux of a penetrating radiation emitted by a source is collimated by a first single-crystal monochromator, the object is irradiated with the collimated radiation, the radiation that has passed through the object is collimated by a second single-crystal and is then recorded by a detector (SU, A, 1,402,871).

According to the method under discussion, the radiation incident upon the object and passed therethrough is effected in an angular range corresponding to the characteristic angles of refraction of the radiation in question effective at the boundary of different-density media of the object under test. Used for collimation are perfect-quality single crystals placed in front of and past the object under test parallel to each other so that the Miller indexes of the reflecting surfaces thereof have the same value and be opposite as to sign.

With the above condition fulfilled the rays that have not refracted when passing through the object and did not deflected from the initial direction set by the first single-crystal monochromator, are reflected from the second single-crystal, which is recorded by the detector in a diffracted beam. At the same time the rays that have deflected from the initial direction at the boundary of two different-density media go beyond the limits of the range of Bragg reflection of the second single crystal so that a drop of the radiation intensity in the detector of the diffracted beam at said interface. Thus, the image of the internal structure of an object is established according to the method discussed above. The method is featured by a higher sensitivity compared with the aforediscussed methods for testing the internal structure of an object against the amount of absorbed radiation and is capable of testing such objects that are characteriezd by weak absorption of penetrating radiation.

However, when the object under test features an boundary between two media, wherein their refractive indices are so close to each other that the angle of deflection of rays at said boundary falls within the range of the angles of Bragg reflection of the second single crystal, such an boundary would not be registered by the detector because the rays deflected at said boundary get onto the detector receiving surface simultaneously with the rays that have been passed through the object without deflection. Thus, the deflected rays do not cause a perceptible reduction of the radiation intensity against the background of the non-deflected rays.

SUMMARY OF THE INVENTION

The present invention is aimed at the provision of a method for obtianing the image of the internal structure of an object, which would make it possible to register on the detector the rays that have deflected at the boundary of two media in the object under test for angles smaller than those in the range of Bragg reflection in the second single crystal, thereby increasing the sensitivity and hence making the method more informative.

The aforesaid aim is attained due to the fact that in a method for obtaining the image of the internal structure of an object, consisting in that the flux of a penetrating radiation emitted by a source is colimated by a first single-crystal monochromator, the object under test is irradiated with said collimated radiation, the radiation that has passed through said object is collimated by a second single-crystal monochromator and is registered by a detector, according to the invention, the object under test is irradiated with asymmetrical refleciton of radiation from a single-crystal monochromator and a width-wavefront pseudosheet-like beam is formed, featuring its divergence in the Bragg diffraction plane at least twice as little as the width of the Bragg reflection range of the second single-crystal, while said second single-crystal is set in a position at which the condition of Bragg reflection with respect to the pseudosheet-like beam formed by the single-crystal monochromator is fulfilled for the reflecting surfaces of said second single-crystal.

It is expedient that for obtaining the maximum image contrast the second single-crystal be rotated, prior to registration, round an axis lying in its plane and being square with the Bragg diffraction plane within the range of Bragg reflection angles, the interference patterns of the internal structure of the object under test be observed at various angles of rotation of the single-crystal and be recorded with the latter single-crystal in a position corresponding to a maximum contrast of the image of the desired area of the object under test.

To enhance the image contrast it is practicable to deflect the second single-crystal from its exact Bragg position in both directions alternatively for equal angles relative to the exact Bragg position in the range of the Bragg diffraction angles, to obtain the images of the object in both of said positions, to perform an algebraic addition of said images, and to record the result of said addition.

It is possible to observe the image of the internal structure of the object in the transmitted and diffracted radiation beams at a time and to record the image which has a higher intensity, whereby a possibility of detecting artifacts in the image is provided.

Whenever the thickness of the second single-crystal is such that less than 10 percent of the penetrating radiation is absorbed therein, the image contrast can be enhanced by performing algebraic addition of the images obtained in the transmitted and diffracted beams and recording the result of said addition.

To eliminate the elements resulting from photoelectric absorption of the radiation by the object under test, the second singlecrystal is first to be deflected for an angle exceeding the range of Bragg reflection angles, whereupon the iamge of the object obtained with the single-crystal in said deflected position is to be algebraically added to the image observed at the point of maximum contrast and the result of addition is to be recorded.

When using a source of hard radiation having a wavelength below 0.3 A, it is expedient, with the purpose of reducing the overall dimensions of the device, that the image be recorded immediately past the output face of the second single-crystal.

The method for obtaining the image of the internal structure of an object, according to the invnetion, is featured by a high sensitivity to low density gradients and is instrumental in obtaining contrast images of internal structure of objects which also carry information on the object interfaces at which the rays of a penetrating radiation have been deflected for angles smaller than the range of Bragg reflection angles of the second single-crystal.

The method, according to the invention, yields essentially best results with zero photoelectric absorption in the material of the object under test. Practical application of the method provides, with a minimum absorbed radiation dose, its high sensitivity and information-bearing capacity which renders the method most promising for use in medicine.

BRIEF DESCRIPTION OF THE DRAWINGS

In what follows the present invention will now be disclosed in a detailed description of some specific illustrative embodiments thereof with referee-ice to the accompanying drawings, wherein:

FIG. 1 is a schematic view of a device for carrying into effect the method for obtaining the image of internal structure of the object under test, according to the invention;

FIG. 2 is a view of FIG. 1 showing an embodiment of the device for use of a hard radiation;

FIG. 3 is a view of FIG. 1 showing a structural diagram of the unit for algebraic addition of images; and

FIG. 4 presents a distribution curve of integral intensity J of a beam reflected from the second single-crystal (reflection curve), wherein plotted as abscissa is the angular deflection Q_(B) of the second single-crystal from an exact Bragg position expressed in terms of units of half-width W_(o) of the distribution curve.

BEST MODES OF CARRYING OUT THE INVENTION

The device for carrying into effect the method for obtaining the image of an internal structure of the image under test, according to the invention, as schematically shown in FIG. 1 comprises a source 1 of a penetrating radiation, e.g., X-rays, slit arrangements 2, 3 forming the beam of the radiation source 1, a single-crystal monochromator 4 placed across the pathway of said radiation beam and so oriented that the radiation emitted by the source 1 is incident upon the input face of the single-crystal monochromator at a grazing angle α. An object 7 under test is placed across the pathway of a beam 5 reflected from the single-crystal monochromator 4 past a slit arrangement 6, said object 7 being set in holders (omitted in the Drawing). Arranged past the object 7 as along the path of radiation is a second single-crystal 8 hereinafter referred to as the crystal-analyzer 8, and a coordinate-sensitive beam detector 9.

According to an embodiment of the device presented in FIG. 2, as distinct from the device illustrated in FIG. 1, the single-crystal monochromator 4 is built up of two single crystals 4-1 and 4-2.

The method for obtaining the image of internal structure of an object, according to the invention, is carried into effect as follows.

A flux of X-rays emitted by the source 1, e.g., an X-ray tube with a Mo anode, is collimated by the single-crystal monochromator 4, using an asymmetric reflection of the radiation from a system of planes (hkl), e.g., (220) of the single-crystal monochromator 4, and a pseudosheet-like beam 5 is shaped, having a broad wavefront and a divergence in the Bragg diffraction plane at least twice as little as the width of the Bragg reflection range of the single-crystal B.

Used as the single-crystal monochromator 4 is a perfect crystal, such as that of silicon, germanium, or quartz, having an area of its effective surface measuring tens or hundreds of square centimeters and a factor of asymmetry in excess of four. In this particular case there is selected such a single-crystal monochromator that has a factor of asymmetry equal to 25. This means, first, that the front of an original beam incident upon the input plane of the single-crystal monochromator 4 will be enlarged by a factor of 25 in the reflected beam 5 and, secondly, that the divergence of the reflected beam 5 will be less than the width of the angular range of Bragg reflection by a factor of √25.

The smaller the grazing angle α of a beam incident upon the single-crystal monochromator 4 the higher the degree of asymmetry of the radiation used and hence the lower the divergence of the pseudosheet-like beam 5 shaped by said single-crystal monochromator 4. The angle of beam grazing is selected in the limits of Q_(B) ≧α>α_(c), where α_(c) is the angle of total external reflection of X-rays from the single-crystal monochromator 4. Concurrently with a reduction of the angular width of X-ray beam incident upon the single-crystal monochromator 4 and formation of a pseudosheet-like wave, the front of said wave is extended. When the degree of reduction of said angular width or the degree of increase of said wavefront is not sufficient for shaping the beam 5 irradiating the object 4, use is made of a block (consisting two or more) of the single-crystal monochromators 4-1, 4-2 as shown in FIG. 2. For instance, when the factor of asymmetry of the single-crystal monochromator 4-1 equals 25 and that of the single-crystal monochromator 4-2 is 30, so the beam 5 will feature a divergence of 0.08".

Under the aforedescribed conditions one or more single-crystal monochromators 4 are to shape the pseudosheet-like beam 5 having characteristics adequate for establishing an interferogram (grating) image of the object 7 irradiated by said beam 5.

The crystal-analyzer 8 is to be so positioned as to fulfil Bragg reflection conditions, for its reflecting planes, with respect to the pseudosheet-like beam 5 shaped by the single-crystal monochromator 4, e.g., the crystal-analyzer 8 is palced in reflection (220).

Used as the crystal-analyzer 8 i-s a perfect crystal free from any macrostresses, dislocations, or second-phase inclusions and having its degree of perfectness high enough not to introduce its own distortions in the image of the object 7.

The crystal-analyzer 8 can be set in a position, wherein its reflecting planes are oriented "for transmission" or "for reflection". The reflecting planes themselves may be both symmetrical and asymmetrical.

The rays incident upon the input face of the crystal-analyzer 8 and belonging to the pseudosheet-like beam 5 shaped by the single-crystal monochromator 4 that have been passed through the object 7 without deflection, and the rays that have undergone microscattering at the boundary of different-density media and imparted an additional phase shift, interfere with the reflecting planes of the crystal-analyzer 8. Thus, participating in the formation of an interference image of the object 7 are both the rays that have passed through the object 7 without deflection and the rays that have deflected at the boundary including those deflected for angles smaller than the angles of collimation performed by the single-crystal monochromator 4 and by the crystal-analyzer 8 situated past the object 7. As a result, an interference image of the internal structure of the object 7 is produced at the output of the crystal-analyzer 8. The interference image thus obtained displays not only the boundaries resulting from a lower radiation intensity (shown on a positive as black areas against a lighter background) due to deflection of the rays at the boundary of the media in the object for angles exceeding the range of the Bragg reflection angles but also bears an information on the boundaries at which the rays have deflected for angles smaller than those of the Bragg reflection range for the crystal-analyzer 8. These boundaries are present in the image of the object 7 as additional black-and-white and white-and-black contrast images.

According to method of the invention, an angular position of the crystal-analyzer 8 is changed, prior to registering the internal structure of the object 7 using the coordinate-sensitive detector 9 transmitting the image to the monitor screen, by rotating said crystal-analyzer about an axis situated in its plane and arranged square to the Bragg diffraction plane, both clockwise and counterclockwise relative the exact Bragg position Q_(B) in the range of the Bragg reflection angles, otherwise speaking within the width of the reflection curve. In this case the image of the boundaries of interfaces in the object which are featured by different density gradients, is observed at a maximum contrast only when the crystall-analyzer 8 assumes a position most suitable for each of said boundaries. Thus, for instance, the angular position of the crystal-analyzer 8 corresponding to the point A in FIG. 4 will be better represent an external contour of the object, whereas its angular position corresponding to the point C will represent an internal structure of the object. An exact Bragg position Q.sub. B of the crystal-analyzer 8 (FIG. 1) practically never corresponds to a maximum contrast of the image of the contours of the object and of its internal structure. By deflecting the crystal-analyzer 8 one is to watch the interferogram images of the internal structure of the object 7 and to select such a position of the crystal-analyzer 8 that gives the image of the parts involved has a maximum contrast.

According to an alternative embodiment of the method, according to the invention, the crystal-analyzer 8 is deflected for equal angles clockwise and counterclockwise with respect to an exact Bragg position Q_(B) in the range of Bragg reflection angles (points C and C' in FIG. 4). The images obtained with the crystal-analyzer 8 in the first and second positions are added algebraically (with opposite signs) and the result of addition is recorded. Such an algebraic addition of similar images differing only in the sign of contrast (that is, one black-and-white and the other white-and-black) makes it possible to substantially enhance the contrast of the image obtained and as a result of such addition.

According to the method of the invention, both of the beams emerging from the crystal-analyzer 8 carry information information on the internal structure of the object 7, that is, the transmitted beam R_(o) and the diffracted beam R_(H). Thus., simultaneous observation of the object in both of the beams R_(o) and R_(H) enables one to detect artifacts, in the image, such as e.g., defects of the recorder or detector. The point is that the image contrast of the same boundary between two media in the object 7 as seen in the beams R_(o) and R_(H) is different for each beam but is obligatorily present in both beams. When the image detected in one of such beams has some elements absent in the image registered in the other beam, this is indicative of the presence of an incidental defect, e.g., that of the emulsion on an X-ray film.

When the thickness of the crystal-analyzer 8 is such that less than 10 percent of the penetrating radiation is absorbed therein, the contrast of the image of the boundaries in the object as seen in the transmitted beam R_(o) and in the diffracted beam R_(H) is as a rule complementary, that is, black-and-white and white-and-black. In this case the images registered in the beams R_(o) and R_(H) are added together with the opposite signs. The contrast of the images of the boundaries in the resultant image is enhanced additionally due to subtraction of information on the areas wherein only photoelectric absorption occurs.

The aforesaid embodiment of the method is instrumental in attaining a higher image contrast without precision displacement of the crystal-analyzer 8.

One more embodiment of the method disclosed herein consists in that the crystal-analyzer 8 is first deflected for an angle exceeding the range of Bragg reflection angles (point D in FIG. 4) to obtain the image of the object 7 with the aforesaid position of the crystal-analyzer 8. The image contains information only on the distribution in the object 7 of areas differing in photoelectric absorption. Once said image has been detected, the crystal-analyzer 8 is set to the position at which the maximum contrast of the object image is attained (e.g., point A in FIG. 4) and the images having opposite signs are added together, after which the image gets rid of the contrast resulting from photoelectric absorption.

When using a hard radiation source featuring its wavelength below 0.3 Å which is the case with medical examination of a living organism, the Bragg angles of reflecting planes are equal to several degrees so that the detector 9 must be spaced a considerable distance (about 1 m) apart from the crystal-analyzer 8 in case the crystal-analyzer 8 is oriented "for transmission" for separate registration of the images of the object 7 in the transmitted beam R_(o) and diffracted beam R_(H). with the resultant considerable increasing of the overall dimensions of the device.

In such a case the image of the object 7 is detected immediately past the output plane of the crystal-analyzer 8 in a zone where the transmitted and diffracted beams R_(o), R_(H), respectively has not yet been practically separated from each other. This makes it possible to obtain contrast images with the same overall dimensions of the device. To this aim the detector 9 is to be situated close to the output plane of the crystal-analyzer 8 as seen in FIG. 2.

FIG. 3 represents one of possible variants of a block-diagram of a device for effecting numerical algebraical addition of images. In addition to the elements described with reference to FIG. 1, the device of FIG. 3 comprises two coordinate-sensitive detectors 9-1, 9-2 of which the detector 9-1 is adapted for detecting the diffracted beam R_(H) and the detector 9-2, for detecting the transmitted beam R_(o). Connected to the output of the detector 9-2 is an analog-to-digital converter 10 to the outputs of which are buffer storage units 11, 12 which in turn are connected, through their outputs, to the inputs of an adder 13. Connected to the output of the adder 13 are series-connected a buffer storage unit 14, a digital-to-analog converter 15, and a video monitor 16. The output of the detector 9-1 is connected, via a feedback unit 17, to the input of the precision displacement actuator of the crystal-analyzer 8.

Each of the pairs of detected images subject to addition is converted by the analog-to-digit converter 10 into a sequence of numerical codes carrying information on the signal amplitude in each of the image elements. The coordinates of the images elements are defined by the position of a code corresponding to a given element in the numerical sequence. The adder 13 carries out an algebraic element-by-element addition of the codes, and the resultant information is put down in the buffer storage unit 14 using likewise the element-by-element technique, whence the information is processed in the digit-to-analog converter 15 and then delivered to the screen of the video monitor 16.

The method, according to the invention, is suitable for testing objects having an amorphous structure, a crystalline structure, or a composition structure of both. The objects under test cna also be of biological origin.

The sole limitation placed upon the object under test consists in that a maximum absorption of a penetrating radiation therein must not exceed 50 percent, otherwise the beam that have undergone microscattering at the interfaces in the object will be absorbed therein so that no interferogram images will be observed.

Thus, use of a pseudosheet-like beam formed by reflection from asymmetric planes of a single-crystal monichromator, for irradiation of the object under examination results in an increased size of the area under examination and in a higher sensitivity of the method to low density gradients, inasmuch as, according to the invention, accuracy of selection of an angular deflection of the detected beams and of its determination is defined not by the width of the reflection curve which is the case with the known method but by the divergence of the pseudosheet-like beam incident upon the object under test, which divergence can be substantially smaller than said width of the reflection curve. It is, due to the aforesaid feature that the contrast at the boundaries of the media having close values of the refractive index of the radiation used is also increased.

A possibility of obtaining a number of images with the crystal-analyzer 8 in different angular positions enables one to get a more detailed knowledge of the internal structure of the object, since the beams that have deflected for different angles at the boundaries are formed in each angular position of the crystal-analyzer 8.

Simultaneous registering of images in two beams, the transmitted and diffracted ones, enables one to define more exactly the reliability of minor and low-contrast image elements and to eliminate errors in interpretation of the image due to imperfection of the image detector. In addition, algebraic addition of said images enhances the contrast of the boundaries between the different-density media, which is due to elimination of a background resulting from photoelectric absorption of radiation in the abject, and by virtue of additional distinction of the boundaries.

The same aim is attained by consecutive registration of images with the crystal-analyzer assuming such angular positions that are deflected for equal angles from exact Bragg position, followed by algebraic addition of said images, and also by consecutive registration of said images with the crystal-analyzer 8 assuming two angular positions, one of which falls within the limits of the reflection curve and the other is outside the limits of that curve.

Registration of a combined image resulting from superposition of images formed in the transmitted and diffracted beams simplifies substantially the image formation process involving the contrasting of boundaries in the internal structure of the object under test, using a hard radiation with small diffraction angles and inconsiderable resolution loss.

The method disclosed herein will find most utility when using powerful radiation sources, in particular, highly penetrating radiation sources, and is applicable for examination of biological objects, ceramics, composite materials, polymer ones inclusive, as well as mixtures of different-density liquids.

Industrial Applicability

The present method for obtaining the image of the internal structure of an object can find application for nondestructive testing of objects made of a variety of materials, including metals, ceramics, composition materials, polymers, and biological objects.

The method is preferably applicable for diagnosis of objects featuring a low degree of absorption of a penetrating radiation and having low gradients of density of its components, e.g., for diagnosing living organisms, or polymer and ceramic products. 

I claim:
 1. A method for obtaining the image of the internal structure of an object, comprising the steps ofdirecting the flux of a penetrating radiation to an object, placing a single-crystal monochromator in the path of the penetrating radiation upstream of said object, a reflected radiation in the form of a wide-wavefront plane beam being formed form said flux of the penetrating radiation with the aid of an asymmetrical reflection plane of said single-crystal monochromator, irradiating said object with the obtained plane beam, placing a second single crystal in the path of the radiation down-stream of said object in a position at which the Bragg reflection of the plane beam incident on said second single crystal is effected, the width of the interval of the Bragg reflection angles of said second single crystal in the diffraction plane being at least twice as large as the divergence angle of the plane radiation beam formed by said first single-crystal monochromator, directing a portion of the plane beam of radiation transmitted through the object and bearing information about the internal structure of the object on to said second single crystal by means of which diffraction of the plane beam of radiation is effected, registering at a detector the plane beam of radiation reflected from the second single crystal.
 2. A method as claimed in claim 1, comprisingturning said second single crystal about an axis square with said diffraction plane by an angle in the range of the Bragg reflection angles, determining the angle of turning which corresponds to the increased contrast range of the image of the investigated portion of the object, and registering the plane beam of radiation reflected from the second single crystal by the detector at the determined angle of turning.
 3. A method as claimed in claim 2, comprising the steps of successively turning said second single crystal about said axis in opposite directions about the exact Bragg position of the second single crystal to the equal angles in the range of the Bragg deflection angles,successively registering first the plane radiation beam reflected from the single crystal when it is rotated in one direction and then in the other direction by said detector, algebraically adding together the images obtained, and registering the result of said algebraic addition.
 4. A method as claimed in claim 1, wherein said detector additionally registers the plane radiation beam transmitted through the second single crystal.
 5. A method as claimed in claim 4, comprising algebraically adding up said two images obtained in registering said plane radiation beam transmitted through the second single crystal, and reflected from the second single crystal, andregistering the result of algebraically adding up the two images.
 6. A method as claimed in claim 2, comprising the steps of additionally turning the second single crystal about an axis square with said diffraction plane up to an angle larger than the range of the Bragg reflection angles,registering by said detector the plane beam transmitted through said second single crystal when the second single crystal is rotated through said angle, algebraically adding up said two images obtained in registering said plane beam reflected from the second single crystal when it is turned up to an angle in the range of the Bragg reflection angles and transmitted through said second single crystal when it is turned up to an angle larger than the range of the Bragg reflection angles, and registering the result of algebraically adding up said two images.
 7. A method as claimed in claim 1, wherein the image obtained is registered immediately in the vicinity of the second single crystal. 